This invention relates generally to automatic package or article wrapping methods and devices. More particularly, it relates to a method and apparatus for heat sealing articles within a thermoplastic film as the articles move at a high rate of speed through an article wrapping machine.
Machines for wrapping articles and packages with a heat sealable thermoplastic film are known art. Indeed, such machines have been utilized commercially for several decades. In a typical configuration, the package wrapping machine advances a steady stream of articles along a conveyer belt and towards a plastic envelope. This plastic envelope is formed by providing a roll of center-folded thermoplastic film which is situated to one side or the other of the conveyer. A continuous sheet of center-folded plastic film is pulled from the roll and is presented along a line which is typically perpendicular to the conveyer and perpendicular to the path of the articles which are moving along that conveyer. Means for separating the adjacent layers of film in the center-folded configuration is provided by use of a film inverter. The film inverter separates and opens the film envelope and reverse folds, or inverts, the film envelope such that the advancing articles are effectively captured by the film envelope and interposed between the adjacent film layers. As the enveloped articles continue their advance, leaving the film inverting area of the machine, one continuous side or edge of the film envelope remains open. A side sealing mechanism is provided for effectively welding or sealing the continuous side or edge of the adjacent and open film layers. Once side sealed, this mechanism provides a generally tube-like thermoplastic envelope for the articles. As the articles and the continuous side sealed plastic film envelope which covers them continue to advance along the machine, an end sealer effectively welds or seals a leading edge of the envelope and then reciprocates to the rear of the article, or simply allows the article to advance, to then weld or seal the trailing edge of the envelope. In this fashion, the leading edge of the next-in-line article is also sealed and the process is repeated.
In the experience of these inventors, one problem which is inherent to the side-sealing action of presently used packaging machines is that incomplete seams or welds often result in the side sealer portion of the machine. This is particularly true if the film tensions are not adequately maintained within the side-sealing portion of the machine as the film is advanced. That is, thermoplastic film which is not properly or adequately grasped within, or advanced by, the side sealer mechanism can result in adjacent film layers which are misaligned or simply not maintained in close enough proximity to one another to form a proper weld or seal as the layers are drawn into the vicinity of a hot wire or weld element. Similarly, adjacent film layers which are not maintained in proper alignment may result in welds which not only appear crumpled or wrinkled, but which are effectively incomplete. If adjacent film layers cannot be brought close enough together during the side-sealing process, it may be necessary to make other adjustments such as increasing the temperature of the weld element, or increasing the time that any given portion of the continuous film layers must remain in welding vicinity to the weld element. This latter adjustment essentially amounts to a slowing down of the packaging process and a net reduction in production.
Another problem which arises is the need to replace the xe2x80x9cconsumablesxe2x80x9d of the packaging machine. In the experience of these inventors, the consumables take two formsxe2x80x94the drive belts of the side sealer mechanism which are functionally intended to grasp and advance the thermoplastic filmxe2x80x94and the weld element itself. In the case of any particular packaging project, drive belt breakage and weld element burn-out are recognized, though unwelcomed, inconveniences. Once broken, drive belt replacement is absolutely necessary for the continued successful operation of the machine. And, suffice it to say that no welds will be made without a properly functioning weld element. The replacement of such consumables, however, is often easier said than done. The reality being that production must come to a halt and, in the case of a broken belt, the belt drive and tail pulleys must be untensioned by use of the proper tools to allow a new belt to be stretched over them and properly seated back into place. In the case of the malfunctioning or shorted out weld element, it too must be replaced by untensioning the weld element fastening means to either end of the weld element by use of the proper tools and by resetting the newly placed weld element to the proper depth and taper so as to allow optimum performance of the weld element. All of this often requires the use of several tools, is time consuming and is surely not something that the production manager looks forward to.
Further in the experience of these inventors, the weld or heat element of the present generation of side sealer mechanisms which are incorporated into packaging machines may also have a tendency to degrade a weld which is, at least initially, completely and acceptably formed. That is, the weld which is formed within the side sealer may leave the welding area in perfect or near perfect condition. If the weld is overexposed to the heat of the hot wire or weld element, by spending too much time near or traveling too close to the wire or element after the weld is formed, the weld quality will be compromised. In extreme cases, the weld may actually be reopened. This too is an unacceptable result in the overall quality of production.
Another problem associated with present packaging machines is that the end sealing mechanisms incorporated in such machines create end seals which may also display weld inconsistencies. One explanation for this, in the view of these inventors, is that the article which is enveloped within the so-called plastic film xe2x80x9ctubexe2x80x9d typically has some height or thickness to it. While some articles are very thin, others are more bulky. For articles with any substantial girth, sealing of the film should optimally occur at or very near to the horizontal midline of the article. In this fashion, an equal amount of film is pulled down around the article from the top film layer as is pulled up around the article from the bottom film layer. This is not, however, how most articles are typically end sealed using machines that are available today. To the contrary, the surface upon which the article rests typically provides the horizontal reference point at which the article is sealed instead.
The present invention overcomes these problems and disadvantages. It provides a new and useful method and apparatus for securely grasping and advancing the adjacent layers of thermoplastic film through the side sealing mechanism of a packaging machine. It also provides a new and useful method and apparatus for maintaining integrity of the heat-induced weld which is created at the side sealing mechanism and at the end sealing mechanism of the packaging machine. It also provides a new and useful method and apparatus which accomplishes all of this while making the side sealing mechanism an apparatus in which the consumables utilized within the mechanism can be maintained or replaced by the user without the need for tools or special adjustments.
The present invention accomplishes this by providing a plurality of film grasping belts which travel about two sets of cooperating pulleys, each pulley set including a drive pulley, a tail pulley and a number of idler pulleys disposed between the drive pulley and the tail pulley. Each pulleyxe2x80x94drive, tail and idlerxe2x80x94has two circumferential belt grooves defined within it, each belt groove being functionally adapted to allow the travel of a belt along it as the pulley is rotated about a central axis. A first line of travel about the pulleys is defined within the cooperating pulleys along a path which is substantially parallel with the line of travel of an article which is being advanced by the wrapping machine. The second pulley line of travel is defined within the cooperating pulleys along a path which diverges away at a slight angle relative to that path of travel. The grooves or pathways of the cooperating drive and tail pulleys of each set are cooperatively tapered or beveled so as to allow this divergence and to prevent the belt from xe2x80x9cwalkingxe2x80x9d out of the grooves. The belts which are used in the method and apparatus of the present invention are V-ribbed belts, the xe2x80x9cV-ribbedxe2x80x9d portion of the belts being disposed outwardly of the pulleys and the flat belt portion being immediately adjacent the pulleys. Each pair of cooperating pulleys is offset by an amount which is equal to one-half of the pitch of one rib and groove of each V-ribbed belt such that the belts engage each other in tooth-meshing fashion. This meshing action is functionally adapted to firmly grasp a pair of thermoplastic film layers therebetween and to prevent slippage therefrom. It should also be noted that the contour of the belt may be varied without deviating from the scope of this invention.
The tail pulley of each pulley assembly is, in the preferred embodiment, mounted in an offset cam fashion and is spring loaded to apply tension to the belts. A rotational handle is also provided. This feature allows torsional pressure to be applied to the tail pulley by the user for quick and tool-less removal of a damaged or broken belt.
Disposed between the divergent belt paths is a hot wire assembly which is functionally adapted to weld the thermoplastic layers together as the plastic film passes near the hot wire. This hot wire assembly is constructed such that the hot wire itself drops down along a line which is at a slight angle relative to the horizontal. It is also constructed such that the hot wire itself is divergent away from the path of travel of the article being sealed. This feature prevents over-exposure of the weld to the hot wire thereby maintaining weld integrity. The hot wire assembly, in the preferred embodiment, utilizes a first wire mounting block which is stationary and a second wire mounting block which is rotatable. A rotational handle is provided. This feature allows torsional pressure to be applied to the rotatable mounting block by the user for quick and tool-less removal of a damaged or burned out hot wire. The mounting blocks of the hot wire assembly are also provided with means for allowing insertion of the hot wire end connectors at a predetermined depth which preserves the preset angle relative to the horizontal and which does away with any need to measure or manually adjust wire depth or angle.
Following the side sealing apparatus of the present invention is an end sealing apparatus which includes an elevation screw for adjusting the end seal location at a point which lies at the middle of the vertical height of the article to be wrapped. The elevation screw has a elevation screw nut which is attached to it, which is in turn attached to a rocker assembly for moving a horizontally disposed and vertically moving top seal bar downwardly and a cooperating bottom seal bar, likewise horizontally disposed and vertically movable, upwardly such that the top and bottom seal bars meet at the vertical center of the article.
In summary, the advantages of the invention are that it provides a method and apparatus for securely grasping and advancing the adjacent layers of thermoplastic film through the side sealing mechanism of a packaging machine; that it maintains integrity of the heat-induced weld which is created at the side sealing mechanism and at the end sealing mechanism; and which makes the side sealing mechanism an apparatus in which the consumables utilized by it can be quickly and easily maintained without the need for tools or readjustment.
The foregoing and other advantages of the method and apparatus of the present invention will be apparent from the description which follows.